Field of the Invention
The present disclosure generally relates to image forming and, more particularly, to an image forming apparatus, an image forming method for making an adjustment on gradation characteristics of an image forming apparatus, and a storage medium.
Description of the Related Art
Generally, a gradation characteristic of an image forming apparatus such as a color printer or a multifunction peripheral is changed because of temporal change, environmental change, or individual variability of the apparatus. In order to maintain gradation of an output document output from the image forming apparatus in a preferable state, techniques known as calibration and gradation correction have been known as the methods for correcting the gradation characteristic that is to be changed because of the above-described factor.
In the techniques, at first, reference patches including a gradation pattern in which gradations of colors used for image formation are gradually changed and criterial patches including a density pattern serving as a criterion are compared visually or compared by using a measurement device. Then, an output characteristic of a color of an image output from the image forming apparatus is corrected by using a comparison result. In order to execute the above correction, visual gradation correction in which an amount of color deviation is visually determined by a user without using a scanner or a measurement device is employed if there is a demand for a simple adjustment method at low cost. In the visual gradation correction, from among the patches (criterial patches) in a plurality of gradations serving as criteria printed on a sample chart, the user selects a patch the density of which conforms to that of a determination patch (reference patch) included in a confirmation chart formed by the image forming apparatus serving as a correction target.
Japanese Patent No. 4697317 discusses a method in which a user visually makes a determination by printing the criterial patches and the reference patches on a same sheet.
In the above-described gradation correction processing of the image forming apparatus, the criterial patches and the reference patches are used at each color regarded as a processing unit of the gradation correction. Further, in order to precisely determine a patch density to improve correction precision, for example, there is provided a known method in which a combination of reference patches in low, medium, and high densities are printed at each color as a gradation correction confirmation chart. At this time, although the number of gradations of the reference patches is optionally specified within a range of 1 to the number that is equal to the number of gradations of the criterial patches, the correction precision is increased if a greater number that is within a range where the user can visually make a determination is specified as the number of gradations thereof. Further, screen processing is generally employed as an image forming method for creating gradations through the image forming apparatus such as a color printer or a multifunction peripheral. Since the color gradation is created according to the number or the density of color dots (color lines) that constitute a screen, the gradation characteristic may vary according to the number or the density thereof. Therefore, it is necessary to execute the gradation correction processing at each screen type. For example, the gradation characteristic becomes different in low line number screen processing and high line number screen processing. Accordingly, the user has to execute the gradation correction processing by using a result of the comparison between the reference patches including respective screens and the criterial patches. Thus, in the above-described gradation correction processing, the number of comparison operations of the criterial patch and the reference patch executed by the user is equivalent to the number of reference patches. Specifically, the number of comparison operations is a value equivalent to a product of the number of colors, the number of screen types, and the number of gradations of the reference patch.
As described above, although a patch configuration of a minimum in which a single patch is used for each determination is employed, the number of reference patches is increased according to the number of colors or screen types. The number of comparison operations is further increased if the number of gradations of the reference patch is increased in order to improve the correction precision.
In the image forming apparatus of a certain printing system, an output density may slightly vary in one printed sheet depending on a position or a peripheral image. For example, in a case where the image forming apparatus is operated in an electro-photographic system, a density difference may occur in a central portion and end portions of a sheet in a scanning direction of a laser, or density deviation may occur in a sub-scanning direction at a rotation cycle of a drum. In a case where the above-described variation of density (hereinafter, referred to as “in-plane deviation”) depending on a position within the sheet occurs in the confirmation chart, the reference patches may not be printed appropriately depending on the printing position. As a result, a correct determination result cannot be acquired from the comparison between the criterial patch and the reference patch, and thus the correction precision will be lowered.
Therefore, in order to absorb a determination error caused by the in-plane deviation, in the gradation correction processing, a plurality of reference patches of the same type are arranged in different positions of the confirmation chart, and a correction value is determined based on a plurality of determination results.
Through this method, a determination error resulting from a printing position of the reference patch can be absorbed. Therefore, in the gradation correction processing for determining a difference between the reference patch automatically read by the measurement device and the criterion, the above method is used as an effective method for reducing the influence of the in-plane deviation. As described above, precision of the gradation correction is improved when the number of reference patches is increased.
However, in the visual gradation correction, because a user visually executes a comparison operation of a reference patch and a criterial patch, a load of the user is increased if the number of comparison operations of the reference patch and the criterial patch is increased.